Hi,
I have a big problem with this Grundig RPC3000.
Sometimes the DC voltage applied to the output transistors appears on the speaker output (22V) and burn the speakers.
This voltage is not present all the time. Sometimes it works fine.
I adjusted the bias current to 32mV, I resoldered the board. It keeps happening.
Which component can be causing this issue? It happens in both channels and it already burnt two pair of good speakers... I cant use it like this. And as it happens randomly I cant diagnose it.
Anyone had this issue before?
Thanks!
I have a big problem with this Grundig RPC3000.
Sometimes the DC voltage applied to the output transistors appears on the speaker output (22V) and burn the speakers.
This voltage is not present all the time. Sometimes it works fine.
I adjusted the bias current to 32mV, I resoldered the board. It keeps happening.
Which component can be causing this issue? It happens in both channels and it already burnt two pair of good speakers... I cant use it like this. And as it happens randomly I cant diagnose it.
Anyone had this issue before?
Thanks!
If this problem is in BOTH channels, it is almost certainly due to the loss of one polarity of the power supply.
If the DC output voltage goes positive, you are losing the negative supply (and vice versa).
The good news, it is likely that nothing in the amplifier was damaged.
Look for a loose DC supply fuse holder, bad connection or solder joint between the supply and the board, etc.
Connect your DVM on the speaker output (use load resistors, not speakers) and wiggle some connections.
If the DC output voltage goes positive, you are losing the negative supply (and vice versa).
The good news, it is likely that nothing in the amplifier was damaged.
Look for a loose DC supply fuse holder, bad connection or solder joint between the supply and the board, etc.
Connect your DVM on the speaker output (use load resistors, not speakers) and wiggle some connections.
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A separate DC protection scheme may be useful to you.
The cause of the malfunction should be looked for according to your description in poor soldering of large parts, violation of PCB tracks, short circuit of wires.40 years is a respectable age. It can be anything, anywhere🙁
GRUNDIG RPC3000 HIFI DE Service Manual download, schematics, eeprom, repair info for electronics experts
The cause of the malfunction should be looked for according to your description in poor soldering of large parts, violation of PCB tracks, short circuit of wires.40 years is a respectable age. It can be anything, anywhere🙁
GRUNDIG RPC3000 HIFI DE Service Manual download, schematics, eeprom, repair info for electronics experts
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"......... It happens in both channels and ......."
does it happen at the same time/simultaneous?
does it happen at the same time/simultaneous?
Thanks for all your repies. They are very useful.
The explanation of when this can be happening (lack of one of the voltages) broguht a lot of light in the analysis.
I will also pay attention to the polarity of the DC voltage on the output. I will double check solder joints and inspect tracks.
The issue is not happening on both channels at the same time. I checked that yesterday. That indicates that common parts whould be fine, but it is a good idea anyway to check fuse holders for corrosion or dirt and all the chain on power supply as it may bring future problems.
Just a thought. Can one of the darlington output transistors be causing this issue? I don't know if they can go bad like this. Generally they open or short or get noisy...
Thanks again.
The explanation of when this can be happening (lack of one of the voltages) broguht a lot of light in the analysis.
I will also pay attention to the polarity of the DC voltage on the output. I will double check solder joints and inspect tracks.
The issue is not happening on both channels at the same time. I checked that yesterday. That indicates that common parts whould be fine, but it is a good idea anyway to check fuse holders for corrosion or dirt and all the chain on power supply as it may bring future problems.
Just a thought. Can one of the darlington output transistors be causing this issue? I don't know if they can go bad like this. Generally they open or short or get noisy...
Thanks again.
Hi all,
I checked for cut tracks and cold solder joints. Everything is ok.
The problem is random in one channel (at least I was able to reproduce the probem in one so far).
The +VDC gets present in the emitter and base of one of the darlington output transistors. When this happens, I turn off the power switch and the -VDC remains in the power supply capacitor forever. I turn the unit on again and the problem is still there, on and off and still there. I knocked on every single component when this happens and the problem does not go away. It goes away when he wants, the same way it appears.
Does anybody have a deep understanding of the circuit and can point out some deffective possible components responsible for this? At this point I should replace every single transistor and capacitor because it is really too risky. It already blew the two beautiful original speakers...
Thanks.
I checked for cut tracks and cold solder joints. Everything is ok.
The problem is random in one channel (at least I was able to reproduce the probem in one so far).
The +VDC gets present in the emitter and base of one of the darlington output transistors. When this happens, I turn off the power switch and the -VDC remains in the power supply capacitor forever. I turn the unit on again and the problem is still there, on and off and still there. I knocked on every single component when this happens and the problem does not go away. It goes away when he wants, the same way it appears.
Does anybody have a deep understanding of the circuit and can point out some deffective possible components responsible for this? At this point I should replace every single transistor and capacitor because it is really too risky. It already blew the two beautiful original speakers...
Thanks.
Attachments
there are some possibilities............perhaps a bad bias pot (R 3016), which loses contact sometimes?
That is a poor design.
The bias pot is in a bad place to keep control of the amp.
If it should become dirty, intermittent, the bias will skyrocket.
The bias pot is in a bad place to keep control of the amp.
If it should become dirty, intermittent, the bias will skyrocket.